Upstream and downstream measurements are used to install, adjust, and troubleshoot amplifiers and other plant components on the CATV network. Traditionally, using a handheld test meter, a test signal of known amplitude is injected onto the network. The test signal propagates through the CATV network, passing through various active and passive components. A technician measures the power level of the test signal with a receiver at various points in the network to isolate problems, adjust active network components, e.g. amplifiers, and install or replace active or passive components, where required.
A sweep test signal extends the test measurements to multiple frequencies. A “reverse sweep” is currently used to test the return plant health at frequencies spaced across the upstream spectrum, i.e. 5 to 45 MHz in the U.S. and 5 to 65 MHz in Europe. A “forward sweep” is used to test the network at downstream frequencies, i.e. 55 to 850 MHz in the U.S. and 75 to 1000 MHz in Europe.
When performing the reverse sweep measurements, care must be taken to prevent interference between the injected test signals and the active services on the network, e.g. DOCSIS, set-top box traffic, and VoIP services. The most straightforward way to avoid interfering with active services is to sweep at different frequencies than those used by the services, which is the approach used by prior-art reverse frequency sweeps. In forward sweeps, since the frequency band is relatively wide, vacant frequency channels within the frequency band can be used; however, this method depends on there being enough unoccupied spectrum channels that the measured frequency response is useful for troubleshooting cable network problems. Unfortunately, as the relatively narrower return path becomes crowded with DOCSIS carriers, the conventional reverse sweep method becomes decreasingly useful. In the extreme case, if there are no empty frequency bands, the conventional method breaks down completely, and no frequency response can be measured without causing code word errors and packet loss in customer's traffic.
When frequency diversity is accomplished with a narrowband transmitter and receiver, synchronization must be maintained between the transmitting equipment and the receiver throughout the course of the measurement. A tradeoff is necessary between sweep time and synchronization precision, which can be challenging when diverse hardware platforms are involved. Closed-loop synchronization further requires two-way connectivity, i.e. instrument access to both the forward and return paths.
Another consideration is that this prior-art approach requires that the transmitter be kept aware of which frequencies are used and unused. In legacy sweep test systems, this is accomplished by the user maintaining a sweep plan, which contains the channel lineup.
In the case of a reverse sweep, time diversity is a possibility because return path signals are bursty in nature, i.e. it is possible to hold off sending sweep pulses until the carriers become idle.
However, when reverse sweeping from anywhere other than the fiber node, one cannot detect upstream activity from the other legs of the network. As a result, one must either anticipate idle times by monitoring the DOCSIS MAP packets, or be informed of idle slots by some headend device, e.g. a CMTS-tied controller, and then make prompt use of the idle time. Both approaches assume that an access to the forward path is available.
Instead of transmitting a test signal, an alternative approach is to use the active signals themselves to characterize the network. A reference measurement is made at the transmitting end of the network under test, the reference measurement is sent to the receiving device, which performs its own measurement. As no transmission is required in the occupied channels, this is an interference-free measurement, again provided that the sweep plan is accurately maintained. Furthermore, if the levels of the active services' signals are varying only slowly over time, precise measurement timing synchronization is unnecessary.
In the case of reverse sweep, application of this method is less straightforward. Because the levels of the various modems in the network exhibit device-to-device transmit level variation, and each device also may exhibit burst-to-burst transmit level variation, obtaining a reliable reference measurement may require referencing bursts from a specific modem, or even specific bursts. Perhaps a greater obstacle is that at the outer edges of the network, or certainly at a specific drop, there may be no modem bursts available to reference. In this scenario, the approach breaks down and requires that one of the other two prior-art approaches to be used.
An object of the present invention is to overcome the shortcomings of the prior art by providing a non-disruptive method of measuring the frequency response of a cable network using signals of low power.